<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body {
            margin: 0;
        }
        div {
            width: 100px;
            height: 100px;
        }
        .a {
            background-color: red;
        }
        .b {
            margin-left: 20px;
            background-color: blue;
            position: absolute;
        }
        .c {
            background-color: green;
            height: 110px;
        }
        .container {
            width: 800px;
            height: 300px;
            margin: 0 auto;
            background-color: #1b6d85;
            position: relative;
        }
        .box {
            width: 800px;
            height: 50px;
            line-height: 50px;
            background-color: rgba(255,0,0,0.5);
            position: absolute;
            bottom: 0;
            left: 0px;
        }
    </style>
</head>
<body>
<div class="a">aa</div>
<div class="b">bb</div>
<div class="c">cc</div>
<div class="container">
    web前端
    <div class="box">box</div>

</div>
</body>
</html>